Atomic pointer writes issue SOLVED.

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dwheelerat_private

SOLVED!

After much searching, I finally found a reference discussing
atomic pointer writes, memory ordering, and Linux.
It turns out that Paul Rusty Russell's
"Unreliable Guide to Locking" (part of the Linux kernel documentation)
specifically notes that it's okay to assume atomic pointer writes
of aligned pointers, AND it discusses how to flush the memory
cache to ensure memory writes are always viewed in order
(the two issues I had).  It even notes a specific case
(Alan Cox does this with some firewalling code).

So, I intend to soon post a new version of the "stacker" module
that by default uses this fast implementation; using this approach,
locks are only needed when state of the stacker is changed
(e.g., adding and removing stacked LSM modules).  I'll also
include a "slower" implementation that has more pleasant support
for removing LSM modules, so you can choose "fast but don't remove"
or "slow and remove at will" through a compile-time flag.

If you use the (default) fast version, to remove a module you first
have to "deactivate" it, and then sometime later remove the module.
Don't remove the module until all threads have had a chance to complete
executing it and walk through it (unfortunately there's no way to detect
this, but that's the price you pay for speed).  I don't expect this
to be a real problem.

--- David A. Wheeler

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